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Publication numberUS4185972 A
Publication typeGrant
Application numberUS 05/891,124
Publication dateJan 29, 1980
Filing dateMar 28, 1978
Priority dateMar 28, 1977
Also published asDE2732491A1, DE2732491C2
Publication number05891124, 891124, US 4185972 A, US 4185972A, US-A-4185972, US4185972 A, US4185972A
InventorsSukekazu Nitta, Takuji Watanabe, Takashi Takahira
Original AssigneeNitta Belt Kabushiki Kaisha
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electric charge holding structure for electretized air-filter medium
US 4185972 A
Abstract
The present invention is an improvement in electretized filtering means. This is an electric charge holding structure wherein the filter medium is covered on two opposite sides with an electrically conductive material having an air permeability that does not impair the air permeability of the filter and these two opposite sides are shorted together. This concept is similar to causing a short in the magnetic lines of force for the purpose of long time retention of the magnetic force as often employed in the vicinity of a permanent magnet. In this invention this idea is applied to electret as an air-filter medium.
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Claims(5)
What is claimed is:
1. An improved electretized filtering means comprising:
an electretized filter medium in a shape having opposite sides;
two covering means, each having a high electrical conductivity and an air permeability at least as great as the air permeability of said electretized filter means, and covering the two opposite sides of said electretized filter medium; and
a shorting means electrically connecting the two opposite side covering means.
2. An improved electretized filtering means as claimed in claim 1, wherein:
said covering means is a vacuum evaporated coating of aluminum.
3. An improved electretized filtering means as claimed in claim 1; wherein:
said covering means comprises a wire sheet.
4. An improved electretized filtering means as claimed in claim 1, wherein:
said covering means is a coating of electrically conductive material.
5. An improved electretized filtering means as claimed in claim 4, wherein:
said material is a material taken from the group consisting of aluminum, silver, copper, brass, iron, stainless steel and carbon fiber.
Description
BACKGROUND OF THE INVENTION

The present invention relates to a means retaining intact the surface charge density on electretized air-filter media for a longer period of time and in high density conditions.

A method for manufacturing a filter medium for air-filters wherein porous sheet material comprising synthetic fiber or the like is electretized is disclosed in Japanese Patent Application Publication No. Sho-51-115234, entitled "Method for electretizing porous sheet material", the applicant being the present applicant. Another disclosure of this sort by the same applicant is in Japanese Patent Application Publication No. Sho-51-116837, entitled "Method for manufacturing a filter medium for air-filters". The electret according to said two disclosures adsorbs fine particles suspended in the air by the coulomb force of the positive and negative charges on its two sides. At the same time, it must meet the same conditions that are required of ordinary air-filter media, such as suitable mesh fineness and low susceptibility to air resistance.

SUMMARY OF THE INVENTION

The present invention is concerned with such air-filter media and is intended particularly for the production of means capable of retaining high surface charge density and high adsorption capacity for a longer period of time.

The concept of this invention is similar to causing a short in the magnetic force lines for the purpose of longtime retention of magnetic force, as often employed in the area of a permanent magnet. The invention applies this idea to electret as an air-filter medium. The invention concerns a structure adapted for long time service as a high performance filter under actual use conditions and is not intended for application to filter media in storage. Needless to say, any filter medium in storage can be satisfactorily preserved only if it is wrapped with aluminum foil or the like.

Therefore, the present invention concerns a structure in which both surfaces are covered with a conductive coating, a metal sheet or the like having the finest possible mesh without impairing the air permeability thereof and in which the lines of electric force from both surfaces covered with such metal coatings, metal sheet or the like are shorted by a suitable number of short circuit lines.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details will be apparent from the following description with particular reference to the accompanying drawings in which:

FIG. 1(a) and FIG. 1(b) show examples of the structure of electretized filter medium according to the present invention;

FIG. 2(a), (b) and (c) show examples of the metal sheet according to this invention;

FIG. 3 shows a performance comparison of a medium having the structure according to this invention and one not having such a structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1(a) as well as FIG. 1(b) shows a structure wherein an electretized filter medium (1) is covered with a metal coating or metal sheet (2) on both sides and wherein the metal coating or metal sheet on both sides is shorted by shorting lines or shorting foil (3).

It is to be noted that the shorting lines (3) or the like must be provided in a suitable number over the entire surface. Especially in the case where a metal sheet is used, shorting lines (3) or the like serve as a means to fix the metal sheet on both surfaces and, therefore, must be of adequate strength. The shorting lines may be in belt form over the entire surface. It is also possible for a metal sheet to be folded in two to enclose a filter medium between, its free ends being fixed with suitable electrically conductive material.

FIG. 1(a) is a typical example of the case where metal coating is used, and FIG. 1(b) is a typical example of the case where a metal sheet is used. Metal coating can be obtained by vacuum evaporation coating or spraying of aluminum or silver-aluminum, and in any case, it is necessary that coating should be applied thinly so as not to lose porosity. The metal sheet may be of the form shown in FIG. 2(a) and 2(c). Various other forms of metal sheet can be used as well. Various kinds of conductors having suitable properties may be used with some variation depending upon whether the coating form or the sheeting form is employed. Among such conductors are aluminum, silver, copper, brass, iron, stainless steel and carbon fiber.

According to the method as above-described, it is possible to obtain an electretized filter medium having a longer service life. At the same time, it is possible to increase the strength of the filter medium itself, especially where it is covered with a metal sheet.

The following examples are exemplary of the method of the present invention:

(1) An electretized filter medium obtained by injecting corona discharge electrons or ions into a non-woven cloth of 6 polypropylene fiber, 500 g/m2, 5 mm thick, was covered with a metal sheet of No. 26 stainless steel, 18-mesh, on both its surfaces, whereby the electric force lines on the both surfaces were shorted. The filter medium was used with a filtering apparatus, and then subjected to long-time air exposure on a dust collection efficiency measuring instrument. Electric potential measurements were done by a rotary sector type instrument. The non-woven cloth was removed from the metal sheet at the time of the measurements. The measurement results are shown in FIG. 3 as curve (a).

(2) An electretized filter medium obtained by injecting corona discharge electrons or ions into a non-woven cloth of 6 polypropylene fiber, 500 g/m2, 5 mm thick, was covered with a metal sheet of aluminum wires aligned at 5 mm intervals (the structure as shown in FIG. 2(c), whereby the electric force lines on both surfaces were shorted. The filter was used with a filtering apparatus, and then subjected to long-time air exposure on a dust collecting efficiency measuring instrument. Electric potential measurements were done by a rotary sector type instrument. The non-woven cloth was removed from the metal sheet at the time of the measurements. The measurement results are shown in FIG. 3 as curve (b).

The performance of an electretized filter medium in which both surfaces were not shorted is shown in FIG. 3 as curve (c). It will be clear from FIG. 3 that in a filter medium having a structure according to the present invention, the surface charge density retained after an initial sharp drop is more than twice as much as that of a filter medium of the other type.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2795290 *Nov 1, 1954Jun 11, 1957Us Rubber CoGas filter
US3118022 *May 22, 1962Jan 14, 1964Bell Telephone Labor IncElectroacoustic transducer
US3449093 *Oct 23, 1965Jun 10, 1969Philip Morris IncLaminated polarets
US4035164 *Nov 29, 1974Jul 12, 1977Minnesota Mining And Manufacturing CompanyMethods for removing charged and non-charged particles from a fluid by employing a pyrollectric filter
Non-Patent Citations
Reference
1 *Electrets-Andrew Germant-Physics Today, vol. 2, No. 3, pp. 8-13, 3-49.
2 *Electrets-Dickinson--Plastics Research Co., Alhambra, California, pp. 1-31, 6/7/49.
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4473382 *Jul 8, 1983Sep 25, 1984Lasko Metal Products, Inc.Air cleaning and circulating apparatus
US4555252 *Mar 28, 1984Nov 26, 1985Dragerwerk AktiengesellschaftElectrostatic filter construction
US4808849 *Feb 3, 1988Feb 28, 1989Canadian Patents And Development Limited/Societe Canadienne Des Brevets Et D'exploitation LimiteePhyllosilicate electrets and a method for their manufacture
US4902306 *Aug 9, 1988Feb 20, 1990Air Purification Products International, Inc.Dual-dipole electrostatic air filter
US5090643 *Mar 15, 1990Feb 25, 1992Spears Morton FForce generating system
US5112677 *Nov 28, 1988May 12, 1992Toyo Boseki Kabushiki KaishaElectret sheet and a method for the production of the same
US5143524 *Feb 20, 1990Sep 1, 1992The Scott Fetzer CompanyElectrostatic particle filtration
US5330559 *Aug 11, 1992Jul 19, 1994United Air Specialists, Inc.Method and apparatus for electrostatically cleaning particulates from air
US5348571 *Jan 8, 1993Sep 20, 1994Metallgesellschaft AktiengesellschaftApparatus for dedusting a gas at high temperature
US5376168 *Jun 5, 1992Dec 27, 1994The L. D. Kichler Co.Electrostatic particle filtration
US5405434 *Sep 22, 1993Apr 11, 1995The Scott Fetzer CompanyElectrostatic particle filtration
US5474599 *Jul 7, 1994Dec 12, 1995United Air Specialists, Inc.Apparatus for electrostatically cleaning particulates from air
US5582632 *May 11, 1994Dec 10, 1996Kimberly-Clark CorporationCorona-assisted electrostatic filtration apparatus and method
US5807366 *Jun 18, 1997Sep 15, 1998Milani; JohnAbsorbent article having a particle size gradient
US5814570 *May 15, 1996Sep 29, 1998Kimberly-Clark Worldwide, Inc.Nonwoven barrier and method of making the same
US5821178 *Nov 6, 1996Oct 13, 1998Kimberly-Clark Worldwide, Inc.Nonwoven laminate barrier material
US5830810 *Feb 20, 1997Nov 3, 1998Kimberly-Clark Worldwide, Inc.Nonwoven barrier and method of making the same
US5834384 *Nov 28, 1995Nov 10, 1998Kimberly-Clark Worldwide, Inc.Nonwoven webs with one or more surface treatments
US5916204 *Jan 26, 1998Jun 29, 1999Kimberly-Clark Worldwide, Inc.Method of forming a particle size gradient in an absorbent article
US5998308 *May 22, 1996Dec 7, 1999Kimberly-Clark Worldwide, Inc.Nonwoven barrier and method of making the same
US6365088Jun 24, 1999Apr 2, 2002Kimberly-Clark Worldwide, Inc.Electret treatment of high loft and low density nonwoven webs
US6537932Oct 8, 1998Mar 25, 2003Kimberly-Clark Worldwide, Inc.Sterilization wrap, applications therefor, and method of sterilizing
US7481234Mar 29, 2004Jan 27, 2009Gustafson Martin KBio-hazard attack family survival dome
US8597405Aug 23, 2011Dec 3, 2013Empire Technology Development LlcSelf-cleaning electret filter
US8809766Jun 28, 2011Aug 19, 2014Empire Technology Development LlcMethods and systems for detecting or collecting particles
EP0443254A1 *Dec 11, 1990Aug 28, 1991The Scott Fetzer CompanyElectrostatic particle filtration
WO2013026193A1 *Aug 23, 2011Feb 28, 2013Empire Technology Development LlcSelf-cleaning electret filter
Classifications
U.S. Classification96/99, 307/400, 55/DIG.39, 55/528
International ClassificationH01G7/02, B01D39/14, G01N1/02, A47L9/10, B03C3/28, G01N1/22
Cooperative ClassificationY10S55/39, B01D39/14, B03C3/28
European ClassificationB01D39/14, B03C3/28